Sulphuric acid is widely used as an industrial reagent and as the electrolyte in lead-acid storage batteries. A sensor for translating acid strength directly into a proportional electric potential can be used to better measure and control both chemical activity and battery charge, especially when the resistance of the sensor is a measure of the temperature of the sensed electrolyte.
Acid concentration and temperature are generally considered the best indications of battery state, so this transducer is for use in an energy gage or charger for battery powered vehicles, fork lifts, uninterruptable power supplies and unattended instrumentation systems.
I considered a number of ways of making a transducer, and in the process discovered that there is about 40 millivolts direct current potential between identical electrodes in electrolytes of differing concentration, corresponding to that in charged and discharged lead-acid storage batteries. While other approaches had some serious difficulty such as instability or sensitivity to temperature or acid, it was learned that with proper selection and treatment of materials, one can construct a reliable and accurate concentration transducer, which is stable with temperature and may be be used when the battery is carrying a large current.
While the electric potential is thermally stable, the electrolytes need to be joined by a capillary tube containing a joining electrolyte, and this will necessarily have an electrical conductance which is highly dependent on temperature. The function will be strong and definite if the container of the joining electrolyte is dimensionally and chemically stable. This is an advantage because the internal temperature of a battery strongly influences the stored charge, safe charging current, and longevity. Moreover, there is good correspondence between electrolyte conductivity and stored charge. Good temperature compensation is obtained with this transducer.
Transducers have been constructed to directly drive an an acid concentration or charge indicator, or a controller. But these are too large for some applications, so a potential amplifier is frequently used, especially when good temperature compensation is required. An unusually cost effective integrated circuit amplifier, based on a new design approach, is shown connected to a small transducer.